The bomb which exploded outside Liverpool Women's Hospital contained ball bearings and could have caused "significant injury or death", counter-terror police have said.
In an update, officers said it was made using "homemade explosive" and the ball bearings "attached to it would have acted as shrapnel".
"Had it detonated in different circumstances we believe it would have caused significant injury or death," said Assistant Chief Constable Russ Jackson.
How or why the device exploded when it did are still being determined, and officers are "not discounting it being completely unintentional".
Mr Jackson added: "It is a possibility that the movement of the vehicle or its stopping caused the ignition."